This invention relates generally to confocal scanning microscopy and optical coherence microscopy. More specifically, it relates to fiber-based optical coherence microscopy systems incorporating a novel, fiber-coupled, angled-dual-axis confocal scanning microscope.
The advent of fiber optics and laser technology has brought a renewed life to many areas of conventional optics. Confocal microscopes, for example, have enjoyed higher resolution, more integrated structure, and enhanced imaging capability. Consequently, confocal microscopes have become increasingly powerful tools in a variety of applications, including biological and medical imaging, optical data storage and semiconductor applications.
The original idea of confocal microscopy traces back to the work of Marvin Minsky. Described in his seminal U.S. Pat. No. 3,013,467 is a system in which an illumination beam passes through a pinhole, traverses a beamsplitter, and is focused by an objective to a focal volume within an object. An observation beam that emanates from the focal volume is in turn converged by the same objective lens, reflected by its second encounter with the beamsplitter, and passes through a second pinhole to a photo detector. The geometry of this confocal arrangement is such that only the light beam originating from the focal volume is able to pass through the second pinhole and reach the photo detector, thus effectively discriminating all other out-of-focus signals.
Contemporary confocal microscopes tend to adopt one of two basic confocal geometries. In the transmission arrangement using two objectives, one objective focuses an illumination beam from a point source onto a focal volume within an object and another objective collects an observation beam that emanates from a confocal overlapping volume (within the focal volume). Whereas in the so-called xe2x80x9creciprocalxe2x80x9d reflection arrangement, a single objective plays a dual role of focusing light on the object and collecting the light emanated from the object. In either case, the confocal arrangement enables the confocal microscope to attain a higher resolution and sharper definition than a conventional microscope, because out-of-focus signals are rejected. This unique ability has made confocal microscopes particularly useful tools in the examination of biological specimens, since they can view a specific layer within a sample and avoid seeing other layers, the so-called xe2x80x9coptical sectioningxe2x80x9d. Confocal microscopy techniques are also exploited to provide a spatial filter in many applications.
The transmission confocal microscope typically employs two separate lenses: one serves as the illumination objective and the other as the observation objective. The single objective in the xe2x80x9creciprocalxe2x80x9d arrangement can also be a single lens, in either simple or compound form. In order to image a thin layer about a few micrometers thick within a sample, however, the numerical aperture (NA) of the objective lenses must be large, so as to provide adequate resolution particularly in the axial direction. This generally results in a short working distance, which is undesirable in practice.
A great deal of ingenuity has accordingly been devoted to improving the axial resolution of confocal microscopes without using high NA lenses. A particularly interesting approach is to spatially arrange two separate illumination and observation objective lenses, or the illumination and observation beam paths, in such a way that the illumination beam and the observation beam intersect at an angle theta (xcex8) at the focal points, so that the overall point-spread function for the microscope, i.e., the overlapping volume of the illumination and observation point-spread functions results in a substantial reduction in the axial direction. A confocal microscope with such an angled, dual-axis design is termed a confocal theta microscope, or an angled-dual-axis confocal microscope, hereinafter. The underlying principle as well as the advantages of confocal theta microscopy are described in the above referenced U.S. patent application Ser. No. 09/628,118, titled xe2x80x9cFiber-coupled, High-speed, Integrated, Angled-Dual-Axis Confocal Scanning Microscopes Employing Vertical Cross-Section Scanningxe2x80x9d of Michael J. Mandella, Mark H. Garrett, and Gordon S. Kino, now allowed, which is incorporated herein by reference for all purposes, and which is hereinafter referred to as xe2x80x9capplication ""118xe2x80x9d.
More specifically, application ""118 discloses an angled-dual-axis confocal scanning microscope comprising an angled-dual-axis confocal scanning head mechanically coupled to a vertical scanning unit. The angled-dual-axis confocal scanning head is configured such that the illumination and observation beams intersect optimally at an angle xcex8 within an object and the scanning is achieved by pivoting the illumination and observation beams jointly using a single scanning element, thereby producing an arc-line scan. The vertical scanning unit further causes the angled-dual-axis confocal scanning head to move towards or away from the object, whereby a succession of arc-line scans that progressively deepen into the object is produced, providing a two-dimensional vertical cross-section scan of the object. The vertical scanning unit also comprises a compensation means, for keeping the optical path lengths of the illumination and observation beams unchanged so to ensure the optimal intersection of the illumination and observation beams in the course of vertical scanning. This novel scanning mechanism, along with the integrated structure of the angled-dual-axis confocal scanning head and the coupling of optical fibers, enables this angled-dual-axis confocal scanning microscope to perform fast and high resolution scanning over a large transverse field of view, while maintaining a workable working distance. The integration of optical fibers and silicon fabrication technology further renders this angled-dual-axis confocal scanning microscope integrity, flexibility, scalability, and maneuverability, as desired in many applications.
For example, one of the applications the aforementioned angled-dual-axis confocal scanning microscope is particularly suited for is optical coherence microscopy (OCM), which effectively filters out multiple-scattered photon noise, thus providing high sensitivity and large dynamic range of detection when imaging in a scattering medium. Although great stride has been made in improving the sensitivity and imaging capabilities of optical coherence microscopy, as exemplified by U.S. patent application Ser. No. 09/042,205, now issued, U.S. Pat. No. 6,201,608, commonly assigned to the same assignee, Optical Biopsy Technologies, Inc. of Santa Clara, Calif., USA, as the present application, optical coherence microscopy has yet to reach its full potential of high resolution and fast scanning, as required in biological and medical applications, particularly in vivo imaging of live tissue which is constantly in motion. Two of the prior art methods of obtaining high axial resolution in an OCM apparatus involve the use of either a large NA objective lens, or the use of a femto-second pulsed laser with a very short coherence length. These methods are described by Wang et al. in xe2x80x9cHigh Speed, full field optical coherence microscopyxe2x80x9d, Proceedings of The SPIE Conference on Coherence Domain Optical Methods in Biomedical Science and Clinical Applications III, San Jose, Calif., January 1999, pp. 204-212, and by Drexler et al. in xe2x80x9cIn vivo ultrahigh-resolution optical coherence tomographyxe2x80x9d, Optics Letters, 21(17), pp. 1221-1223, 1999, all incorporated herein by reference. The primary disadvantage of using a high NA is the limited field of view in which diffraction-limited performance is obtained during high speed transverse scanning. High cost and intricacy of femto-second lasers make the second approach undesirable for a practical instrument.
Hence, there is a need in the art for a new way of applying the techniques of optical coherence microscopy that overcomes the limitations of the prior art methods.
Accordingly, it is a principal object of the present invention to provide an angled-dual-axis optical coherence scanning microscope that achieves
a) high axial resolution;
b) large transverse field of view;
c) long working distance;
d) high-speed vertical cross-section scanning;
e) higher sensitivity and larger dynamic range;
f) improved contrast when imaging in a scattering medium;
g) flexibility and scalability; and
h) simple and low cost construction.
These and other objects and advantages will become apparent from the following description and accompanying drawings.
This invention provides an angled-dual-axis confocal scanning microscope. A first embodiment of the angled-dual-axis confocal scanning microscope of the present invention comprises an angled-dual-axis confocal scanning head and a vertical scanning means. The angled-dual-axis confocal scanning head further comprises a first end of a first single-mode optical fiber serving as a point light source, an angled-dual-axis focusing means, an arc-line scanning means, and a first end of a second single-mode optical fiber serving as a point light detector.
From the first end of the first optical fiber an illumination beam emerges. The angled-dual-axis focusing means serves to focus the illumination beam to a diffraction-limited illumination focal volume along an illumination axis within an object. The angled-dual-axis focusing means further receives an observation beam emanated from an observation focal volume along an observation axis within the object, and focuses the observation beam to the first end of the second optical fiber. The angled-dual-axis focusing means is designed such that the illumination axis and the observation axis intersect at an angle xcex8 within the object, thereby the illumination and observation focal volumes intersect optimally at a confocal overlapping volume. The arc-line scanning means, preferably in the form of a single scanning element such as a silicon micro-machined scanning mirror, is positioned such that it receives the illumination beam from the angled-dual-axis focusing means and directs the illumination beam to the object. The arc-line scanning means also collects the observation beam emanated from the object and passes the observation beam to the angled-dual-axis focusing means. The arc-line scanning means is further capable of pivoting the illumination and observation beams in such a way that the illumination and observation axes remain intersecting at a fixed angle xcex8 and that the confocal overlapping volume moves along an arc-line perpendicular to both the illumination and observation axes within the object, thereby producing an arc-line scan.
The vertical scanning means, in the form of a vertical scanning unit, comprises a vertical translation means and a compensation means. The vertical translation means is mechanically coupled to the angled-dual-axis confocal scanning head, such that it causes the angled-dual-axis confocal scanning head to move towards or away from the object, whereby a succession of arc-line scans that progressively deepen into the object is produced, providing a two-dimensional vertical cross-section scan of the object. The compensation means keeps the optical path lengths of the illumination and observation beams substantially unchanged, so as to ensure the optimal intersection of the illumination and observation focal volumes in the course of vertical scanning. Such a compensation mechanism is also crucial for performing optical coherence microscopy.
Altogether, the first embodiment of the angled-dual-axis confocal scanning microscope of the present invention is designed such that it is capable of performing vertical cross-section scanning in a line-by-line fashion with enhanced axial (i.e., vertical) resolution and greater speed, while maintaining a workable working distance and a large field of view. In applications where a three-dimensional volume image of the object is desired, the object may be further moved incrementally along a transverse direction as each vertical cross-section scan is completed. A plurality of vertical cross-section images thus generated can be assembled into a three-dimensional volume image of a region within the object.
In a second embodiment of the angled-dual-axis confocal scanning microscope of the present invention, an angled-dual-axis confocal head is mechanically coupled to a vertical scanning means and a transverse scanning means. The angled-dual-axis confocal head comprises a first end of a first single-mode optical fiber serving as a point light source, an angled-dual-axis focusing means, and a first end of a second single-mode optical fiber serving as a point light detector.
From the first end of the first optical fiber an illumination beam emerges. The angled-dual-axis focusing means serves to focus the illumination beam to a diffraction-limited illumination focal volume along an illumination axis within an object. The angled-dual-axis focusing means further receives an observation beam emanated from an observation focal volume along an observation axis within the object, and focuses the observation beam to the first end of the second optical fiber. The angled-dual-axis focusing means is designed such that the illumination axis and the observation axis intersect at an angle xcex8 with the object, thereby the illumination and observation focal volumes intersect optimally at a confocal overlapping volume. The vertical scanning means, in the form of a vertical scanning unit, comprises a vertical translation means and a compensation means. The vertical translation means is mechanically coupled to the angled-dual-axis confocal head, such that it causes the angled-dual-axis confocal head to move towards or away from the object, whereby producing a vertical scan that deepens into the interior of the object. The compensation means keeps the optical path lengths of the illumination and observation beams substantially unchanged, thereby ensuring the optimal intersection of the illumination and observation focal volumes in the course of vertical scanning. Moreover, the transverse scanning means, in the form of a transverse stage, serves to translate the object relative to the angled-dual-axis confocal head along transverse directions perpendicular to the vertical direction, thereby providing a transverse scan.
As such, the second embodiment of the angled-dual-axis confocal scanning microscope of the present invention is capable of performing vertical scans and transverse scans in various ways. By assembling an assortment of the vertical and/or transverse scans in a suitable manner, two-dimensional transverse and/or vertical cross-section images of the object can be obtained. A three-dimensional volume image of the object can also be accordingly constructed.
It is to be understood that the term xe2x80x9cemanatingxe2x80x9d as used in this specification is to be construed in a broad sense as covering any light transmitted back from the object, including reflected light and scattered light. It should be also understood that when describing the intersection of the illumination and observation beams in this specification, the term xe2x80x9coptimalxe2x80x9d means that the illumination and observation focal volumes (i.e., the main lobes of the illumination beam""s point-spread function and the observation beam""s point-spread function) intersect in such a way that their respective centers substantially coincide and the resulting overlapping volume has comparable transverse and axial extents. This optimal overlapping volume is termed xe2x80x9cconfocal overlapping volumexe2x80x9d in this specification.
In an angled-dual-axis confocal scanning microscope of the present invention, the angled-dual-axis focusing means generally comprises an assembly of beam focusing, collimating, and deflecting elements. Such elements can be selected from the group of refractive lenses, diffractive lenses, GRIN lenses, focusing gratings, micro-lenses, holographic optical elements, binary lenses, curved mirrors, flat mirrors, prisms and the like. A crucial feature of the angled-dual-axis focusing means is that it provides an illumination axis and an observation axis that intersect at an angle xcex8. The optical fibers can be single-mode fibers, multi-mode fibers, birefrigent fibers, polarization maintaining fibers and the like. Single-mode fibers are preferable in the present invention, for the ends of single-mode fibers provide a nearly point-like light source and detector.
The aforementioned arc-line scanning means typically comprises an element selected from the group consisting of scanning mirrors, reflectors, acousto-optic deflectors, electro-optic deflectors, mechanical scanning mechanisms, and Micro-Electro-Mechanical-Systems (MEMS) scanning micro-mirrors. A preferred choice for the arc-line scanning means is a flat pivoting mirror, particularly a silicon micro-machined scanning mirror for its compact and light-weight construction. Moreover, the arc-line scanning means is placed between the angled-dual-axis focusing means and the object to be examined. This enables the best on-axis illumination and observation point-spread functions to be utilized throughout the entire angular range of an arc-line scan, thereby providing greater resolution over a larger transverse field of view, while maintaining diffraction-limited performance. Such an arrangement is made possible by taking advantage of the longer working distance rendered by using relatively lower NA focusing elements or lenses in the angled-dual-axis focusing means.
A distinct advantage of the angled-dual-axis confocal scanning microscope of the present invention is that the scanning is achieved by pivoting both the illumination and observation beams, as opposed to moving either the object or the microscope""s objective lenses in the prior art confocal theta scanning microscopes, which can be quite cumbersome to implement and adversely limits the precision of scanning.
Another important advantage of the angled-dual-axis arrangement of the present invention is that since the observation beam is positioned at an angle relative to the illumination beam, scattered light along the illumination beam does not easily get passed into the observation beam, except where the beams overlap. This substantially reduces scattered photon noise in the observation beam, thus enhancing the sensitivity and dynamic range of detection. This is in contrast to the direct coupling of scattered photon noise between the illumination and observation beams in a transmission or reciprocal confocal microscope, due to the collinear arrangement between the beams. Moreover, by using low NA focusing elements (or lenses) in an angled-dual-axis confocal scanning system of the present invention, the illumination and observation beams do not become overlapping until very close to the focus. Such an arrangement prevents additional scattered light in the illumination beam from directly xe2x80x9cjumpingxe2x80x9d to the observation beam, hence further filtering out multiple-scattered photon noise in the observation beam. Unfortunately, this arrangement does not eliminate multiple-scattered photon noise that originates within the observation beam. The present invention employs a temporal gating technique, to filter out this source of noise. Altogether, the angled-dual-axis confocal scanning system of the present invention has much lower noise and is capable of providing much higher contrast when imaging in a highly scattering medium than the prior art confocal systems.
A further advantage of the present invention is that the entire angled-dual-axis confocal scanning head in the first embodiment, or the angled-dual-axis confocal head in the second embodiment, can be mounted on a silicon substrate etched with precision V-grooves which host various optical elements. Such an integrated device offers a high degree of integrity, maneuverability, scalability, and versatility, while maintaining a workable working distance and a large field of view. In particular, a micro-optic version of an integrated, angled-dual-axis confocal scanning head (or angled-dual-axis confocal head) of the present invention can be very useful in biological and medical imaging applications, e.g., endoscopes and hand-held optical biopsy instruments.
All in all, the angled-dual-axis confocal scanning microscope of the present invention provides high resolution scanning with greater precision and faster speed, while maintaining a workable working distance and a large field of view. The angled-dual-axis confocal scanning microscope of the present invention further advantageously exploits the flexibility, scalability and integrity afforded by optical fibers and silicon micro-machining techniques, rendering it a highly versatile and modular device.
The present invention provides an angled-dual-axis optical coherence scanning microscope incorporating the aforementioned angled-dual-axis confocal scanning microscope. An exemplary embodiment of the angled-dual-axis optical coherence scanning microscope of the present invention comprises an angled-dual-axis confocal scanning microscope as described above (e.g., in its first or second exemplary embodiment), a light source, a beam-splitting means, a reference optical fiber, and a beam-combining means. The beam-splitting means is in optical communication with the light source and the angled-dual-axis confocal scanning microscope, such that it diverts a portion of an output beam emitted from the light source to the first optical fiber of the angled-dual-axis confocal scanning microscope and a remainder of the output beam to the reference optical fiber, thereby creating an illumination beam and a reference beam from the same parent beam. An observation beam collected by the angled-dual-axis confocal scanning microscope is delivered by way of the second optical fiber of the angled-dual-axis confocal scanning microscope, and is further combined with the reference beam at the beam-combining means to generate coherent interference.
In the embodiment described above, the beam-combining means can be in the form of a fiber-optic coupler, at which the reference and second optical fibers are joined and the reference and observation beams are combined. Balanced detection scheme can be accordingly utilized. The system may further comprise a frequency shifting means optically coupled to the first or second optical fiber of the angled-dual-axis confocal scanning microscope, such that the frequency of the observation beam is shifted relative to the frequency of the reference beam.
Alternatively, the frequency shifting means can be optically coupled to the reference optical fiber, such that the frequency of the reference beam is shifted relative to the frequency of the observation beam. In either case, coherent interference between the reference and observation beams is modulated at a heterodyne beat frequency given by the relative frequency shift between the reference and observation beams, allowing for more sensitive heterodyne detection. Moreover, an adjustable optical delay device may be coupled to the reference optical fiber, the first or second optical fiber, so as to maintain coherent interference between the reference and observation beams at the fiber-optic coupler where they are combined.
In applications where light source has a short coherence length, the optical delay device can be adjusted such that mostly single-scattered light in the observation beam is coherent with the reference beam at the fiber-optic coupler and multiple-scattered light in the observation beam, which traverses over a longer optical path length, does not contribute to the coherent interference, therefore providing further filtering of multiple-scattered light upon detection. To enhance the signal-to-noise ratio of detection, an optical amplifier can be coupled to the second optical fiber, so as to boost up the power of the observation beam returning from the object.
An amplified observation beam has an additional advantage of rendering faster scanning rates and consequently higher pixel rates without appreciable loss in the signal-to-noise ratio, because a shorter integration time per pixel of an image is required in data collection. The implementation of balanced detection in this case allows subtraction of the amplifier noise, since preponderance of the spontaneous emission of the optical amplifier would not occur at the heterodyne beat frequency described above.
The light source in the above embodiment can be an optical fiber amplifier, semiconductor optical amplifier, a fiber laser, a semiconductor laser, a diode-pumped solid state laser, or a broadband OCT light source. The light source may be polarized, or unpolarized. The beam-splitting means can be a fiber-optic coupler, such as an evanescent wave coupler or a fused fiber coupler. Various optical fibers, such as the first, second, and reference optical fibers, are preferably single-mode fibers, for single-mode fibers have the advantage of simplicity and automatic assurance of the mutual spatial coherence of the observation and reference beams upon mixing and detection.
In one case where polarized light is provided by the light source and the beam-splitting means is a polarizing beamsplitter, the orientation of the beamsplitter relative to the polarization of light emitted from the light source can be used to control the ratio of optical power between the illumination and reference beams. Furthermore, the first, second, and reference optical fibers are preferably polarization maintaining (PM) fibers, to control the polarizations of the illumination, observation and reference beams throughout the entire system. The optical coupling between the polarized light source and the polarizing beamsplitter is also preferably by way of a third PM fiber. In this case, the reference and observation beams can be brought into the same polarization by rotating either the reference or second optical fiber before joining them at the fiber-optic coupler. Alternatively, a polarization rotation means, such as a Faraday rotator, can be coupled to either the reference optical fiber or the second optical fiber, such that the reference and observation beams have substantially the same polarization when combined.
The aforementioned embodiment can also be used to provide specific information pertaining to the polarization state of light emanated from a polarization-altering, such as a birefrigent-scattering, medium. Many biological tissues, such as tendons, muscle, nerve, bone, cartilage and teeth, exhibit birefrigence due to their linear or fibrous structure. Birefrigence causes the polarization state of light to be altered (e.g., rotated) in a prescribed manner upon refection. Thus, by detecting induced changes in the polarization state of light reflected from a birefrigent-scattering medium, image representing birefrigent (or other polarization-altering) xe2x80x9cscatterersxe2x80x9d can be obtained. In such a case, the polarizing beamsplitter produces an illumination beam with P-polarization and a reference beam with orthogonal S-polarization from a polarized beam emitted from the light source. An observation beam reflected from a birefirgent-scattering (or polarization-altering) sample carries both P-polarization and S-polarization, where the presence of S-polarization is resulted from the birefrigent (or other polarization-altering) xe2x80x9cscatterersxe2x80x9d in the sample. When the reference and observation beams are combined at the fiber-optic coupler, only the observation beam with S-polarization interferes coherently with the reference beam that has only S-polarization. Consequently, the amplitude of resulting heterodyne beat frequency signal corresponds only to the amplitude of reflectance of light with S-polarization, hence providing an image representing birefrigent (or other polarization-altering) xe2x80x9cscatterersxe2x80x9d in the sample.
In applications where the observation beam with P-polarization is of greater interest, a polarization rotation means, such as a rotatable fiber connector or a Faraday rotator, can be coupled to the reference PM fiber in the above embodiment, such that the polarization of the reference beam is rotated by 90-degree, resulting a reference beam with P-polarization. Upon combining the reference and observation beams in this case, only the observation beam with P-polarization interferes coherently with the reference beam that now has only P-polarization. Consequently, the amplitude of resulting heterodyne beat frequency signal measures only the amplitude of reflectance of light with P-polarization.
Moreover, in applications where both P-polarization and S-polarization of the observation beam are of interest, a first auxiliary polarizing beamsplitter can be optically coupled to the second PM fiber of the angled-dual-axis confocal scanning microscope, serving to separate P-polarization and S-polarization of the observation beam by routing them to fourth and fifth PM fibers, respectively. A polarization rotation means, such as a rotatable fiber connector or a Faraday rotator, can be optically coupled to the reference PM fiber and serves to rotate the polarization of the reference beam by 45-degree, thus rendering the reference beam with both P-polarization and S-polarization. The polarization-rotated reference beam is then delivered to a second auxiliary polarizing beamsplitter, which in turn separates P-polarization and S-polarization of the reference beam by routing them to sixth and seventh PM fibers, respectively. The system may further comprise a frequency shifting means optically coupled to the first or second PM fiber, such that the frequency of the observation beam is shifted relative to the reference beam. The fourth and sixth PM fibers can be further joined by a first auxiliary fiber-optic coupler, where the observation and reference beams with S-polarization are coherently combined and the amplitude of resulting heterodyne beat frequency signal corresponds to the amplitude of reflectance of light with S-polarization.
Similarly, the fifth and seventh PM fibers can be joined by a second auxiliary fiber-optic coupler, at which the observation and reference beams with P-polarization are coherently combined and the amplitude of resulting heterodyne beat frequency signal measures the amplitude of reflectance of light with P-polarization. As such, the exemplary system thus described can be utilized to provide an image pertaining to the birefrigent-scattering (or other polarization-altering) regions in a sample with enhanced contrast.
It should be noted that in the above exemplary cases involving polarized light, a polarization maintaining fiber-optic coupler can be alternatively used with the polarized light source. A polarization rotation means may be optically coupled to the reference PM fiber for rotating the polarization of the reference beam, so as to select the desired polarization of the observation beam. Moreover, an unpolarized light source along with a polarizing beamsplitter can be used to provide a polarized illumination beam and a polarized reference beam with orthogonal polarization. A disadvantage of using an unpolarized light source is, however, that the ratio of optical power between the illumination and reference beams cannot be efficiently adjusted to best suit a particular application.
The present invention further provides an alternative embodiment of an angled-dual-axis optical coherence microscope, comprising a light source equipped with dual output-ports, an angled-dual-axis confocal scanning microscope as previously described, and a reference optical fiber. A first output-port of the light source is optically coupled to the first optical fiber of the angled-dual-axis confocal scanning microscope, transmitting an illumination beam. A second output-port of the light source is optically coupled to the reference optical fiber, providing a reference beam. An observation beam collected by the angled-dual-axis confocal scanning microscope is delivered by way of the second optical fiber, and is in turn combined with the reference beam such that coherent interference is produced for detection.
In the aforementioned embodiment, the reference and second optical fibers may be joined by a fiber-optic coupler, to provide for a balanced detection scheme. The system further comprises a frequency shifting means optically coupled to the first or second optical fiber of the angled-dual-axis confocal scanning microscope, such that the frequency of the observation beam is shifted relative to the frequency of the reference beam. Alternatively, the frequency shifting means can be optically coupled to the reference optical fiber, such that the frequency of the reference beam is shifted relative to the frequency of the observation beam. In either case, coherent interference between the reference and observation beams is modulated at a beat frequency, such that heterodyne balanced detection can be utilized. Moreover, an adjustable optical delay device may be coupled to either the reference optical fiber, or the second optical fiber, to maintain coherent interference between the reference and observation beams at the fiber-optic coupler where they are combined. To enhance the signal-to-noise ratio of detection, an optical amplifier can be coupled to the second optical fiber, so as to boost up the power of the observation beam.
The light source in the above embodiment is preferably a short coherence length source, such as the type commonly used for optical coherence tomography applications. The light source can also be an optical fiber amplifier, a semiconductor optical amplifier, a fiber laser, a semiconductor laser, or a diode-pumped solid state laser, equipped with dual output-ports. Various optical fibers, such as the first, second, and reference optical fibers, are preferably single-mode fibers, for single-mode fibers have the advantage of simplicity and automatic assurance of the mutual spatial coherence of the observation and reference beams upon mixing and detection. If polarized light is provided by the light source, the first, second, and reference optical fibers are preferably polarization maintaining fibers. To ensure that the reference and observation beams have the same polarization upon detection, either the reference or second optical fiber can be rotated by an appropriate amount before they are joined at the fiber-optic coupler.
The angled-dual-axis optical coherence microscope of the present invention, as the above exemplary embodiments demonstrate, offers the advantages of enhanced axial resolution while maintaining a workable working distance, fast and high-precision scanning over a large field of view, while attaining higher sensitivity and larger dynamic range of detection provided by the optical coherence technique. It also advantageously exploits the flexibility, scalability, ruggedness, and low cost afforded by optical fibers. As such, the angled-dual-axis optical coherence microscope of the present invention is particularly suited for applications in which high resolution, high contrast imaging and fast scanning are required, such as in vivo imaging of live tissue for performing optical biopsies in many medical applications.
The novel features of this invention, as well as the invention itself, will be best understood from the following drawings and detailed description.